Moving bed reaction zones have been proposed for a variety of hydrocarbon conversion processes including: reforming, dehydrogenation, isomerization, hydrocracking, hydrotreating, alkylation, and the like. During such hydrocarbon conversion reactions, a carbonaceous material, i.e., coke, is formed and deposited on a catalyst thereby causing it to deactivate. In the moving bed processes, fresh catalyst particles are fed to a reaction zone, which may be comprised of several sub-zones, and the particles flow through the zone by gravity. Coked catalyst is withdrawn from the bottom of the reaction zone and transported to a regeneration zone where a regeneration process is employed to remove the carbonaceous material and optionally perform other steps to restore the catalyst activity. Typically, the catalyst also flows through the regeneration zone by gravity and then is withdrawn from the bottom thereof and transported to the top of the reaction zone.
U.S. Pat. No. 3,647,680 issued to Greenwood et al., discloses a method of operating a continuous reforming regeneration process comprising one or more reforming reactors wherein a moving catalyst bed is employed. The patent further discloses that when multiple reactor systems are employed the reactors are preferably stacked vertically with intermediate heating of the reactant stream between the reactors and have a substantially unbroken column of catalyst particles flowing from the top reactor to the bottom reactor.
U.S. Pat. No. 3,838,039 issued to Vesely et al., discloses a method of operating a continuous hydrocarbon conversion process employing a moving bed reaction and regeneration system. The patent discloses that a variety of hydrocarbon conversion processes such as hydrocracking, hydrotreating, hydrofining, reforming, isomerization, alkylation, dehydrogenation, cracking, and shift gas reaction processes can be performed in the moving bed process. The patent discloses that the length of time that the catalyst shall take to completely pass through the reaction zone can be any period depending upon the type of conversion process, the processing conditions, the degree of catalyst deactivation and the quantity of feedstream passed through the reaction zone, e.g., from a period of about a few hours to in excess of a month. See column 11, lines 30-44, The patent does not, however, provide any specific disclosure relating to the catalyst circulation rate and its effect on the conversion to various products or by-products, i.e., selectivity.
U.S. Pat. No. 4,859,643 issued to Sechrist et al., is directed to a method for regenerating coke contaminated catalyst particles which achieves better utilization of oxygen and minimizes surface area loss of catalyst by confining particles in the combustion section of a regeneration zone to a tapered bed configuration. In the method, catalyst particles move through the regeneration zone in continuous or semi-continuous flow and are formed into a vertically elongated bed of particles in the regeneration zone. An oxygen containing gas is passed through the particle bed in a transverse direction and initiates combustion of the coke deposits along a burn front that extends diagonally through the catalyst bed from the inlet surface of the bed to outlet surface of the bed. The catalyst bed is tapered to reduce the volume of catalyst particles behind the burn wave with respect to the gas flow through the bed thereby increasing the gas flow through the upper portion of the bed and removing catalyst that would be exposed to high temperature flue gas flowing across the bed from the burn front.
U.S. Pat. No. 4,879,424 issued to Harandi, discloses a moving bed catalytic process for converting C.sub.2 -C.sub.10 aliphatic hydrocarbons to high octane gasoline. The patent discloses that the reaction severity can be controlled by employing sequential reaction zones at different temperatures in order to increase to yield of high quality gasoline.
Typically in hydrocarbon conversion reaction such as described above, by-products are formed in addition to the desired product produced by the hydrocarbon conversion reaction. Often, the by-products are formed when the desired product produced by the hydrocarbon conversion reaction is allowed to remain in contact with active catalyst sites on the catalyst. Additionally, by-products can comprise feedstream reactants or intermediate products that are not converted to the desired product because of insufficient contacting with active catalyst sites on the catalyst, e.g., when the catalyst is coked. As noted from the above-identified patents it is necessary to control the moving bed process in order to obtain the desired products. One commonly used control technique is to employ heating or cooling steps at intermediate points within the moving bed reaction zone or between reaction zones. Another technique is to employ multiple reaction zones having different reactor volumes in order to change the residence time or space velocity to achieve the desired conversion.
One hydrocarbon conversion process where both desired products and by-products are formed is the conversion of methanol to products such as olefins and gasoline where a by-product formed during the reaction comprises paraffins. When producing light olefins, i.e., olefins in the C.sub.2 to C.sub.4 carbon range, it is not uncommon that paraffins in the C.sub.1 to C.sub.4 carbon range are also produced.
U.S. Pat. No. 4,499,327 issued to Kaiser, discloses a process for the production of light olefins from a feedstream comprising methanol, ethanol, dimethylether, diethylether, or mixtures thereof, comprising contacting the feedstream with a silicoaluminophosphate molecular sieve at effective process conditions to produce light olefins.
U.S. Pat. No. 4,873,390, issued to Lewis et al., discloses a process for catalytically converting a feedstream, e.g., an aliphatic hetero compound, such as methanol, in to a product, e.g., light olefins, wherein the conversion to olefins can be selectively enhanced as compared to the conversion of to paraffins by employing a catalyst that is not completely regenerated, i.e., contains a desired quantity of carbonaceous material.
U.S. Pat. No. 4,929,780 issued to Wright et al., discloses an integrated process of converting methanol and other lower molecular weights oxygenates to gasoline, distillate range liquid hydrocarbons and ethylene. The patent discloses the use of a fixed bed reactor in conjunction with a fluidized bed reactor in order to increase the yield to ethylene.
In view of the sensitivity of many of the above-described hydrocarbon conversion processes to reaction variables such as temperature, catalytic activity and space velocity, improved processes are sought for controlling the process in order to obtain desired conversion products while inhibiting the conversion to by-products. More specifically, improved moving bed hydrocarbon conversion processes are sought which can provide a sufficient amount of active catalyst sites in order to enhance the conversion to the desired products without enhancing the conversion to by-products.